The proposed research is directed toward an evaluation of the biologic properties of human and rabbit lymphocyte subpopulations in order to more adequately define their role in immunity. Human lymphocyte subpopulations would be separated on the basis of their differential expression of the following cell surface markers: The receptor for sheep red blood cells (E-receptor); another T-specific antigen, T alpha 2; the receptors for complement; the receptor for the Fc portion of Ig: and the receptor for the Fc portion of IgM. These fractionated cells will be evaluated for their ability to respond to a variety of stimuli and to express receptor and/or antigen during stimulation. Rabbit lymphocyte subpopulations will also be separated on the basis of their expression of certain cell surface markers. The relationship of CR and Fc receptor bearing cells will be defined. Other studies will be directed toward determining the biologic properties of the various fractionated lymphocyte subpopulations. In particular the properties of CR+Ig- lymphocyte subpopulation in rabbit appendix will be studied to approach an understanding of its place in B-lymphocyte differentiation.